The invention relates to a method of preheating ladle additions prior to their addition to a melt contained in a ladle. In particular, the preheating of the ladle addition is accomplished by the use of non-transferred electric arc heaters.
In the steel industry, adjustments in the metallurgical composition of the hot metal is increasingly being done in the ladle. The chemistry of the metal is sampled and the necessary adjustments made while in the ladle prior to pouring. Unfortunately, the ladle addition such as alloying materials, gas and other material additions to the melt decrease the melt temperature. To have proper pouring temperature, the hot metal and the ladle is generally superheated to a level above the pouring temperature to compensate for temperature losses associated with the materials being added, cold gas stirring of the bath, and ordinary heat losses to the ambient. One of the principal causes of the temperature drop is the addition of cold lime to the bath for slaging and desulfurization.
In order to avoid the temperature reduction problem associated with ladle additions, three basic approaches have been followed. The first of these is the use of addition materials which will produce exothermic chemical reactions when added to the melt. Examples of this practice can be found in U.S. Pat. No. 4,169,724, issued Oct. 2, 1979 and entitled "Desulfurization of Iron Melts", U.S. Pat. No. 4,342,590, issued Aug. 3, 1982, entitled "Exothermic Steel Ladle Desulfurizer and Methods for its Use", and U.S. Pat. No. 4,357,160, issued Nov. 2, 1982, entitled "Process for Improving the Use of Heat in Steel Production From Solid Iron Material". One disadvantage with these methods is that at least one of the materials to be added must create the exothermic reaction. Also, undesirable reaction by-products may be produced which could result in contamination of the melt.
The second approach to maintaining melt temperature during material addition is the use of combustion burner systems in which the melt additions are directly heated by the combustion flames or by gases which are heated by the combustion flames. However, combustion flames are very inefficient heat transfer devices at typical melt temperatures. The flame temperature (i.e. 2200.degree. C.) is usually only slightly higher than the melt temperature (i.e. 1600.degree. C.). 2000.degree. C. Also, combustion burners typically have oxidizing flames that create oxides of the material being added which places oxygen in the melt. This can result in lower product yields and possible contamination of the melt due to the presence of oxygen or the oxides. Where indirect heating with gas occurs, similar inefficiencies take place. Also, the off-gases that are produced in the ladle are usually at the temperature of the melt. In order to improve operating efficiency, heat recuperation systems are used to recover the energy contained in the off-gases which are exhausted by the ladle.
In comparison to an electric arc heater, large volumes of gas must be heated with the combustion burners in order to transfer the equivalent amount of energy into the added material and ladle. For an air/natural gas combustion system, several times the volume of heated gas is required to transfer the same heat energy that is present in an electric arc heated gas stream. For a combustion system, the size of the recuperation system and pollution control systems which are used to process the off-gases is significantly larger than is required for an electric arc heater. With the combustion system, large volumes of gas are coming in contact with the melt. Because the gas is oxidative and soluble in the melt, this can result in contamination of the metal.
With most additions of material to the melt, the form of the material that is added is usually finely-divided, pulverized or in a powder form. When these materials are heated by the combustion gases, problems arise in the ladle with separating the heated materials from the large volume of combustion gases involved. Additionally, carryover of the added materials with the exhausted off-gases can also occur reducing the amount of material available to combine with the melt as well as adding additional pollutants to the exhausted off-gases.
It would be advantageous therefore to have a method of preheating ladle addition materials that uses a reduced volume of gas as well as providing more effective control of material deposition on the melt.
The third approach has been the use of transferred arc type arc heater to provide the thermal energy directly to the melt. The arc is struck between the electrode and the melt contained in the ladle. Although the thermal energy of the arc is directly transferred to the melt, splashing of the melt, caused by various factors including the arc, onto the electrode interferes with the operation of the arc heater and can damage the electrode. A method where the heating efficiency of the electric arc heater can be retained while substantially reducing the possibility of melt splash onto the electrodes would be beneficial.